This invention relates generally to digital transmission networks and, more particularly, to embedded control of signals transported through distributed elements in digital transmission networks.
Digital transmission networks, such as those based on Synchronous Optical Network/Synchronous Digital Hierarchy (SONET/SDH) standards, are used extensively for transporting broadband communications signals. Network elements, such as multiplexers, digital cross-connect systems, and the like, are used in these transmission networks to support a number of different applications, including some that involve multiple switching functions. One example is xe2x80x9cpath-in-linexe2x80x9d protection switching, also referred to as xe2x80x9cvirtual ringsxe2x80x9d or xe2x80x9cring-on-ringxe2x80x9d, which involves line switching over bidirectional line switched rings (BLSR) and path switching over unidirectional path switched rings (UPSR).
To support these types of applications, network elements include a routing structure, such as a switch fabric, to provide the necessary connections for routing signals through the transmission network. For example, distributed switch fabrics with a segmented control structure are typically used for applications involving multiple switching functions, whereby a separate control domain and separate switch fabric supports each of the separate switching functions. To facilitate the appropriate selection of signals by the distributed switch fabrics, control decisions are often based on the status of the signals, e.g., signal quality. However, existing control structures are known to have limited capability for propagating and utilizing signal status as signals are transported through distributed switch fabrics.
In one type of control structure, signal monitoring may be performed locally at each switch fabric location to derive signal status for the various input signals in order to facilitate switching decisions. Because each switch fabric is controlled by a separate complex control element, the signal status must be xe2x80x9crediscoveredxe2x80x9d at each subsequent switch fabric. Among other problems, rediscovery of signal status adds cost and complexity to the system because signal monitoring must be carried out at each switch fabric location. Moreover, not all types of signal status are capable of being rediscovered. For example, status information indicative of internal system faults or interface faults that occur locally at a given switch fabric does not necessarily propagate forward with the signal to succeeding switching points. As a result, this type of signal status information, which may be useful for subsequent switching decisions, cannot be rediscovered at succeeding switch fabrics.
In SONET-based systems, an alarm indication signal is typically used to alert downstream equipment that an upstream defect has been detected. However, an alarm indication signal is a separate maintenance signal and is not used to retain signal status, e.g., quality information, about a particular input signal. As such, an alarm indication signal does not propagate signal status through the network for individual input signals and, as a result, signal status for each input signal still must be rediscovered at each succeeding switching point using some type of signal monitoring function. In sum, rediscovery does not provide an effective means for resolving a cumulative signal status as the signal propagates through the various switch fabrics.
In another type of control structure, complex control elements for each of the separate switch fabrics may be coupled together in order to facilitate the sharing of signal status information among the various complex control elements. Although this arrangement may alleviate some of the problems otherwise associated with rediscovery of signal status, this arrangement has other disadvantages associated with the close coupling required between multiple control functions within a given complex control element as well as between separate complex control elements. For example, the extensive coordination required between the various control functions within a complex control element and between separate complex control elements results in undesirable switching delays.
Undesirable switching delays, the need for signal status rediscovery, and other problems associated with complex control schemes for distributed switch fabrics are avoided according to the principles of the invention by deriving signal status for each signal as it propagates along a transmission path, embedding signal status in each of the signals so that signal status is carried in at least one of a plurality of signal status layers, and selectively extracting the signal status from any of the layers to facilitate an appropriate selection decision at any switch fabric distributed along the transmission path. Importantly, multiple layers or levels of signal status can be embedded for any particular input signal. As a result, multiple layers of signal status can be used to provide a cumulative signal status of a particular signal as it propagates along the transmission path or to support various combinations of status control, such as each layer being used to track a different level or type of status for the signal.
By embedding multiple layers of signal status in each of the signals and supplying this signal status to control elements directly associated with a switch fabric, the control elements for each of the distributed switch fabrics can be decoupled from each other. Moreover, signal status propagates with each of the signals so that signal status is locally available and directly extractable at each selection point in the transmission path to facilitate the appropriate selection decisions. As a result, signal status does not have to be traced back through previous selection points as in prior arrangements. Control of the signal monitoring functions also becomes less complex according to the principles of the invention because signal status does not have to be rediscovered at all subsequent selection points. Using multiple layers of signal status also allows for a wide range of status control. For example, multiple quality levels or failure conditions can be propagated through the system for any particular input signal, thereby providing a cumulative signal status capability that can be used to support a number of different control requirements in the system.